feat: implement per-enemy damage application (spec §3.2)

- Add applyDamageToRoom() function targeting individual enemies
- Add lowestHPEnemy() helper for focus-fire targeting
- AoE spells distribute damage across all enemies
- Single-target attacks hit lowest HP enemy first
- Refactor processCombatTick spell/equipment/melee/DoT damage to use per-enemy system
- Update golemApplyDamageToRoom in golem-combat pipeline for per-enemy targeting
- Add currentRoom to CombatTickResult and sync through gameStore
- Update combat-actions tests with proper enemy setup for per-enemy tests
- Extract combat-damage.ts module to stay under 400-line limit

src/lib/game/stores/pipelines/golem-combat.ts

@@ -5,7 +5,8 @@
 import { useCombatStore } from '../combatStore';
 import { useManaStore } from '../manaStore';
 import { processGolemRoomDuration } from '../golem-combat-actions';
-import type { ActiveGolem } from '../../types';
+import { lowestHPEnemy } from '../combat-damage';
+import type { ActiveGolem, EnemyState } from '../../types';
 
 export interface GolemCombatContext {
   addLog: (msg: string) => void;
@@ -38,12 +39,33 @@ export function buildGolemCombatPipeline(_addLog: (msg: string) => void): {
 
   const golemApplyDamageToRoom = (dmg: number) => {
     const cs = useCombatStore.getState();
-    if (dmg > 0) {
-      const newFloorHP = Math.max(0, cs.floorHP - dmg);
-      useCombatStore.setState({ floorHP: newFloorHP });
+    const room = cs.currentRoom;
+    if (!room || room.enemies.length === 0 || dmg <= 0) {
+      return { floorHP: cs.floorHP, floorMaxHP: cs.floorMaxHP, roomCleared: false };
     }
-    const roomCleared = useCombatStore.getState().floorHP <= 0;
-    return { floorHP: cs.floorHP, floorMaxHP: cs.floorMaxHP, roomCleared };
+
+    // Golems use focus-fire targeting (spec §9.4) — target lowest HP enemy
+    const target = lowestHPEnemy(room.enemies);
+    if (!target) {
+      return { floorHP: cs.floorHP, floorMaxHP: cs.floorMaxHP, roomCleared: false };
+    }
+
+    const updatedEnemies = room.enemies.map((enemy) => {
+      if (enemy.id === target.id && enemy.hp > 0) {
+        return { ...enemy, hp: Math.max(0, enemy.hp - dmg) };
+      }
+      return enemy;
+    });
+
+    const newFloorHP = updatedEnemies.reduce((sum, e) => sum + e.hp, 0);
+    const allDead = updatedEnemies.every((e) => e.hp <= 0);
+
+    useCombatStore.setState({
+      currentRoom: { ...room, enemies: updatedEnemies },
+      floorHP: newFloorHP,
+    });
+
+    return { floorHP: newFloorHP, floorMaxHP: cs.floorMaxHP, roomCleared: allDead };
   };
 
   return { activeGolems, golemApplyDamageToRoom };